Last month we reported that Barnes & Noble had launched NOOK for Android. It was the first e-reader for Android to support e-book lending and the first app by Barnes & Noble to launch under the NOOK brand. Now Barnes & Noble has updated its e-reader software for other mobile platforms and the PC and has unified its apps under the NOOK brand.

NOOK for iPhone, iPad, and PC will all offer e-book lending as first launched with NOOK for Android. Specific improvements include in-app content rating for the iPad. The iPhone app has also taking some features from the iPad app in regards to customization. NOOK for iPhone will support personalized and professional themes and includes the ability to change the colors of fonts, backgrounds and texts as well as make adjustments to line spacing, font style and sizes. NOOK for iPhone has also been optimized so that e-book cover art and text take advantage of the Retina display on the iPhone 4.

Those who use the NOOK e-reader app on their iPad, iPhone or PC will also appreciate the fact that the last page of an e-book opened and a user’s entire library are synchronized across all software-enabled devices making it easy to flip between platforms and not lose your place.

The e-book lending feature, included with all NOOK e-reader versions, utilizes LendMe technology to allow for an e-book to be shared with other users of the NOOK app or e-reader for up to 14 days.

I love the fact that Barnes & Noble is giving Amazon and its Kindle some serious competition. Having two strong companies in the e-book and e-reader market space has already led to reduced e-reader prices and innovations such as e-book lending. As a result, e-book consumption is taking off as demonstrated by Amazon’s first author to be admitted to it’s Kindle Million Club.

What would be even better would be a strong 3rd competitor, but one doesn’t look to be appearing anytime soon on the horizon. Borders is beginning to make some rumblings with its own e-book store and e-reader app, but the company has some serious room to make up before it can be considered a threat by Barnes & Noble or Amazon. Right now, when it comes to e-books, it looks like it’s just a two-horse race.